Electric filter



1944- A. J. DEVOT v 2,355,516

ELECTRIC FILTER Fled Sept. 3, 1941 SPIRAL LY WOUND CONDENSER SWAGED AND SOLDERED 2 r 2 10 20 FR0uENcr FREQUE/VCV- MEGACYCLEJ Jag-5 1 23-6 Patented Aug, 8, 1944,

Emc'rnrc FILTER Andre I. Dvot, Newton, Mam, assignor to Tobe Deutschmann Corporation, Canton, Mass., a corporation of Delaware Application September 3, 1941, Serial No. 409,414

This invention deals with electrical wave filters and especially with filters for use in connection with electrical power machinery, although some of its characteristic features are applicable to filters generally, and is a continuation-in-part of application'se rial 1941.

It is one of the principal objects of the invention to provide a wave filter adapted to eliminate No. 381,867, filed March 5,

from a power transmission circuit comparatively high parasitic frequencies, such as of the signaling and connnunication ranges, but to pass direct current and alternating current of power transmission frequency, and having a minimum of parts, yet including all electrical elements characterlstic of wave filters of the more flexible ,types.

In one aspect of the invention, the new filters provide maximum attenuation of all undesirable frequencies with a minimum loss of energy of power transmission frequency. they can be matched closely to the'impedances of the load and of the source of undesirable frequencies as for example a generator, and are yet mechanically simple and rugged and comparatively inexpensive in manufacture and maintenance, as compared in this respect with the systems with which they are combined. These conditions which seem a good deal contradictory are reconoiled according to the invention by building up an. electrically easily controllable and flexible circuit well suited for the purpose at hand, as for example 01' the series-derived M-type, from a ,M-type filter circuit to obtain maximum periormance .for the purpose in question, together with the simplest possible mechanical structure constituting that circuit. In this circuit, the series impedance is inherently small if, as for the p rpose in question, the series coil or coils are to resonate far above the band used, so that their series capacity is negligibleand the losses of the power alternating and direct currents are small, which again fits into the general purpose of design: the attenuation curve is favorablefor the purpose in question, it tapers slowly on the high 11 (Cl. rum-44) widen the attenuation band; the attenuation can be kept high by selecting a high value for the characteristic ratio of series impedance tofour times shunt impedance; and the relative values of shunt inductance and shunt capacitance which provide thedesirable resonance frequency for the purpose in question are such that they fit the electrical characteristics of the mechanical structure according to the invention, namely comparatively small series and shunt inductances. The shunt inductance being provided. ac-

cording to the invention, by' the positively and exactly determinable condenser impedance, this construction and circuit provide an eflicient high attenuation filter at comparatively low cost.

In a further aspect, the invention provides a and dimensioning of the condenser inductance. In this respect, one of the principal features of the invention is a spirally wound condenser whose foil portions are swaged and broadly soldered to a coil or ground plate, so that the condenser inductance depends merely on the length of the capacitance, these values lending themselves well to the construction of a series-derived M-type filter in which, according to the invention, the impedance values can be exactly controlled by practically eliminating lead shunt inductsnces side and hence permits cascading in order to u and series capacitances.

Some of these aspects are set forth and claimed in my copending application Serial No. 381,867, filed March 5, 1941, for Electric filters, of which this is in part a continuation.

The above and further objects, aspects and features will be apparent from the following detailed explanation illustrating the genus of the invention with reference to several concrete embodimerits thereof. This description refers to a drawin! in which Fig. 1 is a diagrammatical view ofa filter section according to the invention:

Figs. 2, 4 and 7 are the equivalent circuits of filters according to the invention;

Figs. 5 and 6 are front elevational' and plan views, respectively, of a two section cascaded illter according to the invention; and

mass and 8 are attenuation curves of filters accordingtol 'igalandt, respectively.

Fig. lindicatesahalfsectionmadeaccording to the invention, it being of course understood that T and w' sections, unbalanced or balanced, canbemadeupanalogously. Inthisfigurc, iis a condenser of the spirally wound type having metal foil plates or electrodes and a paper dielectric. The foils are, according to common practice, brought out at the ends of the condenser roll, and swaged together. 'Ihe inductance coil 2 is, according to the invention, swaged or soldered at 3 directly to one plate formation,

the other plate being'similarly joined at 4 to a conductor or ground plate I.

The circuit of this structure is shown in 1'18. 2, in which 21 is the series impedance and a the shunt impedance. These impedances are selected to conform to the well known principles defining series-derived M-type filters, namely 1i h) where 21 4a:

In these relations, Li and Is are the series and shunt inductances, respectively; C1 and C: the series and shunt capacitances, respective I the cut-off frequency; r the attenuation peak frequency; a the attenuation constant; and F indicates the functional relationship between a and 4a In the present instance, since in the neighborhood of fan the absolute value of 2a is small, In will have to be much smaller than L1. The construction according to the invention avoids any shunt inductance element proper but utilizes for and lute value as well as the resonance frequency value of the shunt impedance as a whole can therefore be easily determined. Since any series inductance coils of the network must resonate far above the band used so that the distributed capacity C1 is negligible, it follows that the circuit contains only the values L1.Is and Since further, the present mechanical construction is especially suited to provide these impedance values, it will be evident that the invention provides with a minimum of structure a precision type filter which is for its purpose optimallyeffective.

purpose. v

Generally. speaking, a filter of this type will s,sss,s1s

have acircuitasshowninrlg;4,withseries impedances Li andIa' andshuntimpedancesh, Ca and Is, Cs. However, if the high frequency section is put ahead of the lower frequency section, the calculated value of L1 becomes negligibly small. Therefore, and since cascaded ll-derived filters do not require precedence by M-derived sections, a second series coil can be omitted.

- This simplifies thecircuit and lowers the losses at power transmission frequencies. Figs. 5 to 8 illustrate a filter of this type.

InFigs. 5 and 6, numerals ii and i2 denote condensers of the-type described with reference to Fig.- 1 and swaged or soldered at one side to a base plate II and on the other side to the ends I, ii of glass insulated coil il. Hie base plate I! is folded in order to form a shield 2| which also supports two condenser clips 22, fl and a bolt holding the coil between insulating disks 2., 21.

The equivalent circuit of this type of filter is shown in Fig. 7, and its attenuation curve in Fig. 8. The electrical values of an actually built 5- kva. can be accommodated in an oil tank of about 2%" x 2% x 3" overall dimensions. which clearly indicates the compactness of the novel construction.

It will now be evident that one of the main features of the present invention, namely the 'pe-' culiar elimination of lead impedance and control of capacitor inductance, can be applied to other than M-derived filter circuits. although that par- 40 that this invention'includes all modifications and that purpose the condenser inductance which depends only upon the capacitor length: its absoticular circuits lends itself especially well to combination with this mechanical construction.

It should be understood that the present disclosure is for the purpose of illustration only and equivalents which fall within the scope of the appended claims.

I claim: 1. An electric filter of the type having an inductive series impedance and a shunt impedance consisting of inductance and capacitance in series, comprising a condenser having residual inductance connected as shunt impedance and a V coil connected as series impedance, the electrodes so of said condenser being substantially directly connected to said coil so as to constitute a lead impedance which is negligible in the operating range of the filter, and the inductance and capacitance values of said condenser being pro- 53 portioned to constitute said shunt impedance.

2. An electric filter of the series-derived M- type for attenuating radio frequencies and passing low frequency and directpower currents, comprising a condenser having residual inductance connected as shunt impedance and a coil connected as series impedance, the electrodes of said condenser being substantially directly connected to said coil so as to constitute a lead impedance which is negligible in the operating range of the filter, and the inductance of said condenser being selected to constitute, together with the capacitance of said condenser, the shunt impedance value conforming to the characteristics of said series-derived M-type.

3. An electric filter comprising an individual condenser connected as shunt impedance and an individual coil connected as series impedance, said condenser being spirally wound apart from said coil, the electrodes of said condenser being substantially directly connected to said coil so as to constitute a lead impedance which is negligible in the operating range of the filter, and the residual inductance of said condenser being selected to conform to a predetermined resonance frequency value.

4. An electric filter of the type having a comparatively small series impedance and a shunt impedance consisting of inductance and capacitance in series, comprising a shunt condenser having residual inductance and a series coil, the impedance value of said coil being so small that its capacitancevalue vanishes, the electrodes of said condenser being substantially directly connected to said coil so as to constitute a lead impedance which is negligible in the'operating range oi the filter, and the inductance and capacitance values of said condenser being proportioned toconstitute said shunt impedance.

5. An electric filter with one filter section having a series impedance of negligible value and a shunt impedance, and cascaded thereto a second section having an inductive series impedance and a shunt impedance, said shunt impedances consisting of inductance and capacitance in series, comprising two condensers having residual inductance connected as, shunt impedances and a coil connected therebetween as series impedance, the electrodes of said condensers being sub-I stantiallydirectly connected to said coil so as to constitute lead impedances which are negligible in the operating range or the filter, and the inductance and capacitance values or said condensers' being proportioned to constitute said shunt impedances.'

6. An electric filter or the series-derived M- type for high attenuation in a radio frequency range comprising two condensers having residual inductance connected as shunt impedances and a coil connected therebetween as series impedance, the electrodes of said condensers being subbeing substantially directly connected to said coil so as to constitute'lead impedances which are negligible in the operating range of the filter,

' and the residual inductances oi said condensers being selected to conform to predeterminedresonance frequency values.

' 8. An electric filter comprising two shunt condensers of unequal capacity, and connected between said condenser a series coil, said condensers being of the spirally wound type with metal, foils protruding at one end and being solidly connected together and directly to said coil thereby substantially eliminating lead impedance between condensers and coil, and the height and windings of said condensers being dimensioned to provide a predetermined resonance frequency value.

9. A radio frequency band elimination filter comprising a pair of condensers having compare-- contained therein and an inductance oi a larger stantially directly connected to said coil so as to constitute lead impedances which are negligible in" v the operating range of the filter, and the inductances of said condensers being selected to con-. iormto constitute together with the capacitances oi the condensers the shunt impedance values.

conforming to the characteristics of said seriesderlved M-type.. v

'1. An electric filter comprising-twoindividual condensers connected as shunt impedances and an individual coil connected as series impedance, said condensers being spirally wound apart from said coil, the electrodes of said condensers value connected with substantially negligible lead impedance in the line between said capacities, said capacities forming withtheir residual inductances the shunt members of the M-typ filter whose series member is'said larger inductance. p

11. A radio frequency M-type filter of the type described, comprising a line having an'input and an output terminal, a conductive casing having a condenser base constituting an input and output terminal, a pair of condensers with one electrode of each condenser directly connected to said base and the other electrodes directly connected to said line, and an inductance of approximately the value of the sum of the residual inductances or the condensers connected in said line between said condensers, said condensers forming with their residual inductances the shunt members or the M-type filter whose series member is said larger inductance.

' ANDRE J. na'vo'r. 

